In nuclear medicine, images of internal structures or functions of the body are acquired by using gamma cameras to detect radiation emitted by a radio-pharmaceutical that has been injected into the patient's body. A computer system controls the gamma camera to acquire data and then processes the acquired data to generate the images. Nuclear medicine imaging techniques include single-photon emission computed tomography (SPECT) and positron emission tomography (PET). SPECT imaging is based on the detection of individual gamma rays emitted from the body, while PET imaging is based on the detection of gamma ray pairs emitted in coincidence in opposite directions due to electron-positron annihilations. Accordingly, PET imaging is sometimes referred to as coincidence imaging.
One factor which has an impact on image quality in nuclear medicine is non-uniform attenuation. Non-uniform attenuation refers to the attenuation of radiation within the body before the radiation can be detected. Such attenuation tends to degrade image quality. A technique which has been used to correct for non-uniform attenuation is transmission scanning. In transmission scanning, gamma radiation is transmitted through the patient to a scintillation detector and used to form a transmission image, i.e., an attenuation map. The attenuation map provides an indication of the amount of attenuation caused by various structures of the body and can, therefore, be used to correct for attenuation in the emission images.
Another factor which affects image quality is deadtime loss. Deadtime relates to the inability of a gamma camera system (the detector, the electronics, or both) to distinguish between two radiation-induced scintillation events occurring very close together in time. Deadtime loss can be defined as the difference between the true countrate (the "singles rate") and the measured countrate due to deadtime. In an ideal system, in which there is no deadtime loss, the measured countrate would equal the true countrate. In contrast, in a real system, which is subject to deadtime loss, the measured countrate is lower than the true countrate.
Some deadtime correction techniques rely upon approximations of deadtime losses and are therefore inherently subject to inaccuracies. In particular, deadtime losses are dependent, at least in part, upon the singles rate; as the singles rate increases, deadtime losses tend to increase. In transmission studies, because the singles rate varies spatially due to many factors, such as the size and shape of the patient, deadtime losses also tend to be spatially dependent. Correction techniques which rely upon approximations typically do not account for such variations. Another known technique for correcting for deadtime loss is to apply a single, global correction factor to the acquired data, assuming the detector is operating in a limited range of singles rates. The use of a global correction factor, however, has the same disadvantages as mentioned above, i.e., it fails to take into consideration the spatial dependency of deadtime losses. Other deadtime correction techniques rely upon precise calibration of the gamma camera system. However, calibration may be time consuming, subject to inaccuracies, and based on incorrect assumptions. Moreover, calibration also does not take into account differences in size and shape between patients. Hence, it is desirable to provide a deadtime correction technique which overcomes these and other disadvantages of the prior art.